Generally, this kind of rear bicycle derailleur is provided with a fixing member, a movable member movable axially with respect to the fixing member, a chain guide having a guide pulley, a tension pulley and pulley plates, with the chain guide assembly being and supported swingably to the movable member, and a tension spring interposed between the movable member and the chain guide. When a speed control lever is forwardly operated it makes the movable member via a control wire toward a maximum tooth number low speed sprocket against the bias of a return spring. When the lever is backwardly operated it moves the movable member toward a minimum tooth number high speed sprocket by use of a restoring force of the return spring. The chain guide is stopped at the position corresponding to a desired sprocket to thereby switch thereto the chain.
A cyclist, especially, an unskilled cyclist, may insufficiently operate the operating lever and stop the chain guide before the center line of the width of the desired sprocket, resulting in an insufficient movement of the chain guide.
The chain guide at the conventional derailleur, as shown in FIG. 4, has a pair of pulley plates a and b supported swingably to a fixed shaft e at the movable member d through a tubular pivot shaft c fixed to one pulley plate. A bearing cylinder f is supported between the pulley plates a and b, and a guide pulley g is supported rotatably onto the outer periphery of bearing cylinder f. A screw thread h is formed on the outer periphery of the end of pivot shaft c, and a nut n tightly screws with the screw thread h, so that the other pulley plate b is axially moved to hold the bearing cylinder f between the pulley plates a and b and also to restrain the guide pulley g from axial movement.
Therefore, when an amount of movement of the chain guide is insufficient, the chain having engaged with the desired sprocket skews between the guide pulley and the sprocket in engagement with the chain. Hence, the skewed portion of the chain often strikes the exit portion for the chain at the guide pulley and the teeth at the entrance portion for the chain at the sprocket, thereby generating loud noises.